In a continuation of studies on the dynamics of phosphate metabolism in neoplastic cells, we are investigating the properties and activities of inorganic pyrophosphatase (E.C. 3.6.1.1) in a wide range of normal rat tissues and in various rat neoplasms, prinicipally hepatic tumors of varying degrees of growth rate and differentiation. Methods are being developed for its quantitative assay in crude systems, separation and identification of multiple forms by electrophoresis, purification of the enzyme in certain tumors and studies of its chemical and kinetic properties. This study is intended to broaden our insight into possible growth control factors involved in the formation and turnover of pyrophosphate in the regulation and maintanence of cellular levels of inorganic phosphate and nucleoside phosphates.